1. Field of the Invention
Embodiments of the present invention pertain to non-chromated corrosion inhibiting primer formulations, which are useful for structural bonding applications. More particularly, non-chromated corrosion inhibiting primer formulations are provided, which include a releasable organic and/or inorganic corrosion inhibitor.
2. Description of the Related Art
Corrosion of bonded metal parts is a major concern in the aerospace and other high performance industries. Historically chromated primers have been used to protect metals from corrosion. However, due to new REACH and OSHA regulations, the use of chromates will be restricted by 2010 in the aerospace industry, among others.
The corrosion protection mechanism for chromates is well documented in literature. U.S. Pat. No. 5,951,747 reports that chromates perform the following four functions to have excellent corrosion protection on various metal alloys:                a) Releases easily from the polymer matrix to the corrosion site;        b) The chromate ion adsorbs readily on to bare metal. This alters the space charge distribution at the metal-primer interface, lowering the isoelectric point of the protective metal oxide layer, which naturally forms on active metal. This repels the corrosive ions and shifts the corrosion potential of the metal to more noble state;        c) It forms adherent chromium oxide layer at cathodic sites and blocks further corrosion reaction; and        d) It helps to neutralize the increasing acidity at the metal-electrolyte interface thus lowering corrosion. Increasing the acidity accelerates corrosion exponentially.        
Several non-chromated corrosion inhibitors such as zinc phosphosilicates, molybdenum zinc phosphate, calcium borosilicate, sodium vanadate, strontium phosphate etc. have been under evaluation. Most of these inhibitors are passive (cannot leach-like chromates) and provide corrosion protection by sacrificial oxidation method.
Conventional organic corrosion inhibitors are discussed in Kuznetsov Y. L, et al., “Organic Inhibitors of Corrosion of Metals,” Pleumb Pub Corp., 1996. U.S. Pat. No. 6,933,046 reports that a mechanism through which organic species prevent corrosion is by reacting with the metal substrate, the oxide film or the corrosion products to form an adherent film to prevent further corrosion. It also reports that highly effective corrosion inhibitors interact with metal by chemical adsorption.
A major drawback of these organic corrosion inhibitors relates to the interaction of the functional groups used to form strong adherent bonds on a metal substrate with the primer formulation. Due to this interaction, the shelf life and cure kinetics of the primer may be affected, which limits corrosion inhibitor transport within a coating to the corrosion site. Another drawback with many organic corrosion inhibitors is their unpredictable corrosion performance when used with epoxy based corrosion inhibiting primer formulations in preventing corrosion on highly corrosive material such as Al-2024.
Thus, non-chromated corrosion inhibitors that perform similarly to chromate primers for structural bonding application in high performance industries such as aerospace require further improvement. Identification of organic corrosion inhibitors for use in primer formulations having a sufficient shelf life and suitable cure kinetics, while maintaining corrosion and inhibitor transport within a coating to the corrosion site would be a useful advance in the art.